/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMeshOverlay.h
Insert element at given index, assuming it is unused. If bUnsafe, we use fast id allocation that does not update free list. You should only be using this betweenBeginUnsafeElementsInsert()/EndUnsafeElementsInsert()calls
[EMeshResult](API\Runtime\GeometryCore\UE__Geometry__EMeshResult) InsertElement ( int ElementID, const RealType * Value, bool bUnsafe )